Frank McGarrity, a young lawyer in Princeton, takes the case of William, who won't give his last name but carries plenty of cash. William confides he murdered a man but assures him he won't kill again. He doesn't admit that he longs to be the most famous serial killer ever and plans to execute thirteen privileged people who took advantage of his indigent mother or him.
McGarrity is torn between legal ethics and moral right. As a lawyer, he must keep any secrets as long as a client doesn't reveal an intent to commit a future crime.
After visiting Biscuit Buckingham, a jailed bank robber client who was caught because he stopped to help a dog, McGarrity finds out his legal mentor, Jack McCabe, has been killed by an arrow in the back. He shadows William and realizes he's scoping out his next victim, reporter Cindy Halsey. Back home he discovers his fiancée Trish is having an affair with a doctor. He moves out of their apartment into Jack's vacant one.
Biscuit escapes from jail in William's laundry truck. William murders a nun and Doctor Benjamin, Trish's lover. McGarrity concludes he's the only one who can stop the "bow sniper."
William coerces Biscuit into robbing another bank and abandons him after the job. While Biscuit manages to elude the police in huge Mercer County Park, William executes his grade school teacher.
Biscuit meets with McGarrity in a Trenton cemetery. After Biscuit tells him where William lives, he convinces Biscuit to plead guilty to bank robbery and cooperate against William. That will force the court to relieve him as William's counsel due to a conflict of interest on the bank robbery charge.
McGarrity drives to William's hideout and finds a scrapbook of victims. He goes to the home of Elaine, the next target. Although with her two children she's about to separate from her husband, McGarrity persuades her to tell the authorities about her ex-boyfriend—Ernest McGinty, who is using the alias William. William finds out from a utility worker that McGarrity's been in his house, and he aims to kill him.
William's house is raided but he's not there. McGarrity takes Cindy cross-country skiing in Mercer County Park. William stalks them, and McGarrity leaves Cindy to act as a decoy. Biscuit, returning to his park hideout, hears William's shouts. He wounds him with the gun of his nephew, another of William's victims.
William escapes from the jail infirmary out an unbarred window. McGarrity tries to warn Cindy, but she refuses to talk to him for abandoning her in the park. Figuring that William cached some weapons at his abandoned childhood home, he races there to find them first. Neither weapons nor William are there, and he walks down the street to check on Elaine's grandfather, who'd taught William archery. William, having assaulted Gus to steal his bows and arrows, ambushes McGarrity. Again McGarrity is spared, this time with Elaine's help. Although stunned by a blow to the head, William dives out a picture window and escapes.
McGarrity retreats to his apartment where Trish dropped in earlier that day. She's suffering from depression, has fled her home and overbearing mother, and wants to pick up with McGarrity where they left off. He wants no part of it, although he's willing to care for her until he can get her safely back to her family. William barges in while McGarrity's trying to feed McCabe's three-legged dog, Darrow.
I violated the privilege. My luck has run out….
